1. Field of the Invention
The present invention relates to base station apparatuses and radio communication methods used in radio communication systems.
2. Description of the Related Art
A conventional base station apparatus is explained first. FIG. 1 shows a block diagram showing the configuration of a conventional base station apparatus. FIG. 2A and FIG. 2B are schematic drawings showing transmission models in a mobile communication environment. FIG. 4A and FIG. 4B are schematic drawings showing reception directivity patterns of an adaptive array antenna receiver.
Transmission models in radio communications are explained using FIG. 2A and FIG. 2B. Here, by way of example, a case where the number of antennas of radio communication apparatuses (base station apparatuses) 21 and 30 is 3 is explained. FIG. 2A shows the uplink (from a terminal to a base station) and FIG. 2B shows the downlink (from the base station to the terminal).
A signal transmitted from terminal apparatus 25 is reflected by building 26 or mountain 27, etc. and reaches antennas 22 to 24 of base station apparatus 21. Such a transmission path is called xe2x80x9cmulti-path transmission path,xe2x80x9d and the technology for compensating multi-path transmission is called xe2x80x9cequalization technology.xe2x80x9d Generally, failing to compensate this multi-path transmission results in deterioration of the communication quality. In this example, suppose that a signal from the building is received with an extremely small delay (generally, one symbol or less). A signal from mountain 27 containing a large delay may therefore be a major factor of deterioration of the communication quality. On the other hand, signals transmitted through antennas 31 to 33 of base station apparatus 30 are reflected by building 36 or mountain 37, etc. and reach antenna 34 of terminal apparatus 35.
In order to suppress multi-path transmission, it is desirable to transmit a signal through transmission path 39. In multi-path transmission paths, the communication quality of transmission path 40 and transmission path 41 changes as the terminal moves. In this case, it is important to detect the direction of an optimal communication quality and carry out transmission focused on that direction.
The operation of the conventional base station apparatus shown in FIG. 1 is explained. Here, by way of example, a case where the number of antennas of the base station apparatus is 3 is explained. Signals received through antennas 1 to 3 are passed through antenna sharing devices 4 to 6, respectively, amplified, frequency-converted and A/D-converted by radio reception circuit 7 and baseband signals or IF signals are extracted from there. If transmission and reception signals have a same frequency (TDD transmission), switches are used instead of sharing devices.
These signals are sent to timing detection section 8. Timing detection section 8 calculates optimal reception timing. The optimal reception timing is calculated, for example, by embedding a pattern known to both the transmitter and receiver in a frame, transmitting it from the transmitter, carrying out A/D conversion by the receiver for a period several or over ten times one-symbol time, performing correlation operation with the known symbol and detecting timing t0 at which great power results from the correlation operation.
This timing t0 is sent to decimation section 9. The decimation section sends the received signal of timing t0 to adaptive array antenna receiver 12. The adaptive array antenna receiver combines the received signals from the three antennas in such a way that the desired wave or SIR reaches a maximum value. The combination result and weighting coefficients used for multiplication of the received signals of the antennas are output. These weighting coefficients form reception directivity. FIG. 4A gives an example of reception directivity. As seen from the drawing, the reception directivity gain in directions shown by arrows 201 and 202 is large, while the gain in the direction shown by arrow 203 is small. FIG. 4B plots the direction (angle) on the vertical axis and reception directivity gain on the vertical axis.
An adaptive array antenna receiver is explained in xe2x80x9cWaveform Equalization Technology for Digital Mobile Communicationsxe2x80x9d (pp.101-116, published by Triceps Corporation on Jun. 1, 1996, ISBN4-88657-801-2), etc.
Performing adaptive array antenna processing to extract a desired signal provides directivity for the desired signal, producing a portion with small directivity (called xe2x80x9cnullxe2x80x9d) 203 in an unnecessary signal (signal in the same direction as that of the desired signal which arrives at a different time because it takes a different transmission path, or signal from another transmitter). The number of null points is known to be (number of array antennasxe2x80x941) and when the number of antennas is 3, two null points are formed as shown in FIG. 4A and FIG. 4B.
Then, the transmitting side is explained. Modulator 13 modulates transmission data. Sum-of-products calculator 14 multiplies (generally, complex multiplication) the data by a weighting coefficient which has a same directivity pattern based on the reception weighting coefficient. Generally, since TDD (Time Division Duplex) transmission uses a same radio frequency for both transmission and reception, the same reception weighting coefficient is used for multiplication as the transmission weighting coefficient.
On the other hand, in FDD (Frequency Division Duplex) which uses different radio frequencies between transmission and reception, the directivity pattern will be different if the same reception weighting coefficient is used, and therefore a transmission weighting coefficient is re-created and multiplied based on the weighting coefficient above. The re-creation of the transmission weighting coefficient above is explained in xe2x80x9cSpectral efficiency improvement by base station antenna pattern control for land mobile cellular systemsxe2x80x9d by Ohgane in TECHNICAL REPORT OF IEICE (RCS93-8, 1993-05), etc.
Then, the result of multiplying the weighting coefficient to obtain the same directivity as the reception directivity through re-creation of the transmission weighting coefficient is subjected to frequency conversion and amplification by radio transmission circuit 15 and transmitted from antennas 1 to 3 via antenna sharing devices 4 to 6.
By way of example, FIG. 4C shows the transmission directivity. The drawing shows that the transmission directivity gain is large in the directions shown by arrows 204 and 205, and the signal is transmitted with the same directivity pattern as the reception directivity shown in FIG. 4A. The explanations hereafter will omit the re-creation of the transmission weighting coefficient due to differences in radio spectrum between transmission and reception.
Thus, transmitting signals with the same directivity pattern as the reception directivity pattern based on the reception signal weighting coefficient combined by the adaptive array antenna has the following advantages:
(1) Avoiding transmission in the direction in which an unnecessary signal has arrived allows the transmitting side to compensate the multi-path transmission path. This eliminates the necessity for providing the receiver (terminal side) with a high-level technology such as equalizer.
(2) Avoiding transmission in the direction in which an unnecessary signal has arrived limits the reach of the transmitted radio wave, making it possible to improve the spectral efficiency of the downlink.
(3) Using the reversibility of transmission paths, signals are transmitted through transmission paths on the uplink with greater power of the desired wave or with greater SIR, and thus the power of the desired wave or SIR is greater on the downlink, too.
However, the conventional system has the following problems:
(1) When signals are transmitted on the downlink with the same directivity pattern as that on the uplink, the directivity is directed in directions other than null points (directions in which the desired wave may be included) because of directivity pattern characteristics, which means that transmission power is dispersed in those directions. This may cause the reception power of the desired wave of the communication counterpart (terminal) to deteriorate compared to when signals are transmitted only in a specific direction.
(2) Because of directivity pattern characteristics, the directivity is also directed in directions in which the desired wave may be included, and thus interference with other stations may be greater compared to when signals are transmitted only in a specific direction with an optimum SIR among reception directivity patterns.
Furthermore, in a CDMA system using spread spectrum communications:
(3) Compared to when signals are transmitted only in a specific direction with optimum power of the desired wave or SIR, interference with other stations is greater, and thus the system capacity deteriorates.
It is an objective of the present invention to provide a base station apparatus capable of preventing deterioration of the reception power of desired radio waves, reducing interference with other stations and at the same time avoiding deterioration of the system capacity.
This objective is achieved by a base station apparatus comprising a reception section for performing adaptive array antenna reception, a first directivity formation section for forming reception directivity from the received signal, and a second directivity formation section for forming new directivity from the reception directivity above which is limited to a specific direction.